1. Field of the Invention
The invention is drawn to a method and apparatus for managing the irrigation of plants.
2. Description of the Prior Art
Automatic irrigation scheduling triggered by canopy temperature and time thresholds has been successful in trials for corn, cotton and soybean (Evett et al. 2006. Controlling water use efficiency with irrigation automation: Cases from drip and center pivot irrigation of corn and soybean. pp. 57-66 In Proc. 28th Annual Southern Conservation Systems Conference, Amarillo Tex., Jun. 26-28, 2006; Peters and Evett. 2008. Automation of a center pivot using the temperature-time-threshold method of irrigation scheduling. J. Irrig. Drainage Engr. 134 (3): 286-290). Key attributes of such automatic irrigation scheduling are the control of crop water stress and water use efficiency by delivering irrigations only when required and by applying the necessary amount of water without compromising yield or quality.
The crop water stress index, CWSI, has been related to leaf water potential (Howell et al. 1984. Evaluation of Cotton Canopy Temperature to detect crop water stress. Trans. ASAE 27(1): 84-88; Ben-Asher et al. 1992 Canopy temperature to assess daily evapotranspiration and management of high frequency drip irrigation systems. Agric. Water Manage. 22(4): 379-390; Jackson. 1991. Relationships between normalized leaf water potential and crop water stress index values for acala cotton. Agric. Water Manage. 19(2): 135-149; Oliva et al. 1994. White clover seed production: I. Crop water requirements and irrigation timing. Crop Science 34(3): 762-767; Cohen et al. 2005. Estimation of leaf water potential by thermal imagery and spatial analysis. J. Exp. Botany 56(417): 1843-1852), linked to soil water content (Idso and Reginato. 1982. Soil- and atmosphere-induced plant water stress in cotton as inferred from foliage temperatures. Water Resources 18(4): 1143-1148; Colaizzi et al. 2003. Estimating soil moisture under low-frequency surface irrigation using Crop Water Stress Index. ASCE J. Irrigation and Drainage Engr. 129(1): 27-35), used to characterize crop water stress (Idso et al. 1981. Normalizing the stress degree day for environmental variability. Agric. Meteorol. 24: 45-55; Allen and Nakayama. 1988. Relationship between crop water stress index and other physiological plant water status indicators in guayule. Field Crops Research 18(4): 287-296; Yazar et al. 1999. Evaluation of crop water stress index for LEPA irrigated corn. Irrig. Sci. 18(4): 171-180; Yuan et al. 2004. Evaluation of a crop water stress index for detecting water stress in winter wheat in the North China Plain. Agric. Water Manage. 64(1): 29-40; Moeller et al. 2007. Use of thermal and visible imagery for estimating crop water status of irrigated grapevine. J. of Exp. Botany 58(4): 827-838), and evaluated as a tool for irrigation timing (Throssel et al. 1987. Canopy temperature based irrigation scheduling indices for Kentucky Bluegrass turf. Crop Science 27(1): 126-131; Nielsen. 1990. Scheduling irrigations for soybeans with the Crop Water Stress Index. Field Crop Research 23(2): 103-116; Garrot et al. 1994. Quantifying wheat water stress with the crop water stress index to schedule irrigations. Agron. J. 86(1): 195-199; Gontia and Tiwari. 2008. Development of crop water stress index of wheat crop for scheduling irrigation using infrared thermometry. Agric. Water Manage. 95(10): 1144-1152). This thermal based index provides a relative measure of plant stress which can be derived from radiant leaf temperatures and ambient meteorological parameters (Pinter et al. 1983. Infrared thermometry: A remote sensing technique for predicting yield in water-stressed cotton. Agric. Water Manage. 6(4): 385-395). The theoretical CWSI developed by Jackson et al. (1981. Canopy temperature as a crop water stress indicator. Water Resources Research 17: 1133-1138) incorporated incoming solar radiation, relative humidity, air temperature, wind speed, canopy resistance at potential evapotranspiration, and crop height. Its general form is given as:
  CWSI  =                    (                              T            c                    -                      T            a                          )            -                        (                                    T              c                        -                          T              a                                )                ll                                      (                                    T              c                        -                          T              a                                )                ul            -                        (                                    T              c                        -                          T              a                                )                ll            where Tc is crop canopy temperature, Ta is air temperature, (Tc-Ta)11 is the lower limit representing the temperature difference for a well watered crop, (Tc-Ta)u1 is the upper limit representing the temperature difference between the crop canopy and ambient air when the plants are severely stressed (Jackson et al. 1988. A reexamination of the crop water stress index. Irrig Sci. 9: 309-317). The CWSI tends towards 0 after irrigations and progressively increases towards 1 as soil water is being depleted.
However, despite these and other advances, the need remains for improved methods for controlling irrigation, particularly for plants grown under low rainfall and semi-arid conditions.